The fifth generation of mobile telecommunications and wireless technology is not yet fully defined but in an advanced draft stage within 3rd Generation Partnership Project (3GPP). It includes work on 5G New Radio (NR) Access Technology. Long term evolution (LTE) terminology is used in this disclosure in a forward looking sense, to include equivalent 5G entities or functionalities although a different term is specified in 5G. A general description of the agreements on 5G NR Access Technology as of November 2016 is contained in 3GPP Technical Report 38.802 v0.3.0 (2016-11).
In 3GPP, there are past and ongoing study items and work items that look into a new radio interface for 5G. Terms for denoting this new and next generation technology have not yet converged, so the terms NR and 5G will be used interchangeably.
One of the first major decisions that the 3GPP TSG RAN WG1 needs to take for NR concerns is what is often denoted with the terms “numerology” and “frame structure”. In 3GPP TSG RAN WG1, the term numerology is used to determine important numeric parameters that describe aspects of the OFDM radio interface, such as subcarrier spacing (SCS), OFDM symbol length, cyclic prefix length, number of symbols per subframe or slot, subframe length, and frame length. Some of these terms could also fall under the term frame structure, such as e.g. frame length, number of subframe per frame, subframe length, and location and number of symbols in a slot, frame or subframe that carry control information, and location of channels that carry data. In NR a subframe is 1 ms and establishes a 1 ms clock. Transmissions use slots or mini-slots. A slot consists of 7 or 14 symbols, 7 symbols for subcarrier spacings less than or equal to 60 kHz and 14 symbols for subcarrier spacings greater than 60 kHz.
In addition, the term frame structure can comprise a variety of additional aspects that reflect the structure of frames, subframes and slots, for example the positioning and density of reference signals (pilot signals), placement and structure of control channels, location and length of guard time for uplink to downlink switching (and vice versa) for time division-duplexing (TDD), and time-alignment. Generally, numerology and frame structure encompass a set of fundamental aspects and parameters of the radio interface.
LTE supports a single subcarrier spacing of 15 kHz. For some other parameters in LTE, there is some additional flexibility. For example, it is possible to configure the length of the cyclic prefix and the size of the control region within a subframe. Similarly, LTE can support multiple different frame structures, e.g. for frequency division-duplexing (FDD), TDD, and Narrowband Internet of Things (NB-IoT), respectively.
3GPP TSG RAN WG1 has recently agreed that that it shall be possible support mixed subcarrier spacing on the same carrier in NR. The feasibility of mixed subcarrier spacing was studied e.g. in 3GPP contribution R1-163224, where it was shown that the interference between non-orthogonal subcarriers can be mitigated successfully.
Downlink Control Channel
For NR a proposed frame structure and DL control channel structure is shown in FIG. 3. The first OFDM symbol(s) contains at least Physical Downlink Control Channel (PDCCH). The set of OFDM symbols carrying PDCCHs is known as the control region. The length of the control region may be fixed, be semi-statically configured, or dynamically signaled. Following the OFDM symbol(s) with the control region, the data and Demodulation Reference Signal (DM-RS) start.
PDCCH to one particular user is carried on a subset of OFDM subcarriers. The mapping of PDCCH can either be distributed or localized. In localized mapping, a control channel element (CCE) is formed by resource elements (REs) within a same physical resource block (PRB) pair. In distributed mapping, a CCE is formed by REs within two or more PRB pairs. For simplicity the illustration in FIG. 3 is localized.
PDCCH can carry among others downlink scheduling information indicating DL resources in the same (or also later) slot. FIG. 3 shows two PDCCH and the corresponding two scheduled Physical Downlink Shared Channel (PDSCH), corresponding to upward hashing and horizontal hashing. In addition, a third PDCCH is shown without corresponding PDSCH, e.g. an UL grant, corresponding to downward hashing. A UE detects PDCCH addressed to it and derives from it relevant control information, such as scheduling information. The figure illustrates the case of a control region size of one OFDM symbol. In case the control region extends over multiple OFDM symbols, the mapping of PDCCHs could be done such that one PDCCH is not restricted to a single OFDM symbol but is allowed to span multiple OFDM symbols in the control region (multi-symbol PDCCH mapping). Alternatively, the mapping can be such that one PDCCH is transmitted in one OFDM symbol only (multiple PDCCHs may be transmitted in one OFDM symbol (per-symbol mapping). Normally, one PDCCH is associated with a specific radio network temporary identifier (RNTI). Because a RNTI may be associated with a specific UE, a group of UEs or all UEs of a cell, one PDCCH may be directed to one UE, to all UEs in a cell or to a subgroup of the UEs in the cell.
Per-symbol PDCCH mapping has the benefit of resulting in a time-division multiplexing structure, i.e. PDCCHs in different OFDM symbols can be beamformed in different directions with (analog) beamforming. Multi-symbol PDCCH mapping, on the other hand, may provide benefits in terms of e.g. frequency diversity (different parts of the frequency domain in different OFDM symbols may be used by one PDCCH) and power setting.
Beamforming
Beamforming is a multi-antenna technique to concentrate radiated or received energy into a few directions. At lower frequencies digital beamforming can be performed where the combining of received signals across antenna elements is done in digital domain (on a receiving side) or where the transmit antenna element weights are set in digital domain (on a transmitting side). In a multi-carrier system such as OFDM, beam weights are typically set in frequency domain, i.e. before the transmitter IFFT or after the receiver FFT in an OFDM system. This implies different weights can be applied on different bandwidth portions of the carrier and thus different beams can be realized on different bandwidth portions of the carrier. With digital beamforming it is thus possible to create within one symbol multiple beams pointing to multiple directions/users.
With analog beamforming the beamforming weights are set in the analog domain, either at RF frequency, some intermediate frequency or even baseband. The analog signal is in the time domain, i.e. any formed beam is the same across the complete carrier. With analog beamforming a beam typically covers only one or few users (since in the single beam area only few users are located). To schedule (i.e. point a beam to) multiple users beam sweeping is typically required over a train of (not necessarily contiguous) symbols, each symbol transmitted with different beam weights (and thus beams) covering different users. While with digital beamforming multiple users can be addressed with a single symbol analog beamforming typically requires multiple symbols, see FIG. 4.
Analog beamforming is typically applied in millimeter-wave (mmW) frequencies where large bandwidths are available and many antenna elements are utilized. Processing of large bandwidth requires very fast analog-to-digital converter (ADC) and digital-to-analog converter (DAC) equipment, which are expensive and have relatively high power consumption. In digital beamforming one ADC/DAC is needed per antenna element (group) while in analog beamforming one ADC/DAC is needed per beam and polarization layer. Analog beamforming, where the beamforming weights are applied downstream of the DAC on the transmitting side, is thus simpler and may consume less power, especially for large bandwidth and many antenna elements, such as in the mmW range.
Sending control information to a single user does typically not require a complete OFDM symbol (i.e., the full bandwidth of the symbol is not required), especially at mmW frequencies where considerable bandwidths are available. However, typically only one or few users are within a beam, leading to underutilized OFDM symbols if each OFDM symbol cannot be filled with information to the users within the beam. To reach multiple users with analog beamforming beam sweeping across multiple symbols is applied, and each symbol is potentially underutilized.
Prior art therefore proposes to use wider subcarrier spacings for the DL control channel, relative to the data channel, see FIG. 5. By applying wider subcarrier spacing for the DL control channel, each OFDM symbols becomes shorter and includes fewer subcarriers (for a given bandwidth, an OFDM symbol using 2n×15 kHz carries ½n as many subcarriers as a 15 kHz symbol does). Accordingly, each OFDM symbol is better utilized and the train of symbols gets shorter. However, wider (and thus shorter) subcarriers for DL control is not needed in all systems, e.g. systems using digital beamforming can address multiple users within one symbol. A UE trying to connect to the network does not know which OFDM numerology (e.g. subcarrier spacing but also cyclic prefix length) the network applies for the DL control region.
Furthermore, different PDCCH mappings (per-symbol mapping or multi-symbol mapping) may be preferable in different scenarios.
It is therefore a problem for the UE to know which OFDM numerology is used for the DL control channel and what PDCCH mapping to use.